A method of measuring the permeability of hollow fiber membranes by means of a transient diffusion experiment was developed and demonstrated in our previous work. This method has been made more efficient and flexible by developing (1) a semi-continuous concentration monitoring system which is compatible with several methods of concentration measurement and (2) a procedure which requires the preparation of only one membrane sample per solute. A second goal is to use the transient diffusion experiment to study diffusion in blood at mass-transfer rates comparable to those encountered in the artificial kidney. This experiment involves the transient diffusion solutes as urea and creatinine through various blood components enclosed by sealed hollow fiber membranes. BIBLIOGRAPHIC REFERENCES: J.F. Stevenson, "Permeability Measurements for Hollow Fiber Membranes," (Copy submitted to Elias Klein for inclusion in NIH book, "Evaluation of Hemodialyzers and Hemodialysis Membranes," Feb. 1977), in press.